The present invention relates to an applanation tonometer for measuring the tonometric or intraocular pressure of an eye by applanation of its cornea. It is clinically important for the ophthalmologist to be able to accurately measure the intraocular pressure of a patient's eye in order to screen and treat various forms of potentially blinding conditions ranging from glaucoma to ocular hypoteny. The principals of applanation tonometry, as presented by Goldman, are the basis for almost all modern slit lamp tonometers which are constructively based upon and utilize the fact that pressure is equal to force multiplied by area. In applanation tonometry, force is applied via a small tonometer tip to a surface of the cornea until the force is sufficient to flatten a known area of the corneal surface. The area of the tonometer tip is designed such that the required force in dynes multiplied by ten is equal to the intraocular pressure in millimeters of mercury. The area used is 3.06 mm. and is achieved as an end point by utilizing a fluorescein stained tear layer under blue cobalt light illumination to highlight the compressed area of the cornea. Under these conditions, the area of the cornea that is compressed is dark blue while the tea layer immediately surrounding the tonometer tip compression is highlighted bright green. The diameter of the circular tonometer tip is set optically via a prism in the tip which splits the highlighted circular area into two overlapping semicircles. A force varying knob of the tonometer is adjusted until the resultant bright green semicircles are partially overlapped.
The tonometer tip is viewed through one-half of the a slit lamp bimicroscope of the slit lamp tonometer and, once properly applanated, is withdrawn from the surface of the cornea by moving the entire slit lamp assembly away from the patient. A force/pressure scale of the tonometer is then read independently of the optics of the slit lamp by the operator gazing down at the force/pressure scale which appears on one of the force varying knobs of the tonometer. A typical applanation tonometer of the type just described is disclosed in U.S. Pat. No. 3,470,736 issued to Stephen A. Bartfay on Oct. 7, 1969 and entitled OCULAR TONOMETER and in U.S. Pat. No. 3,070,997 issued to Franz Papritz et al. on Jan. 1, 1963 and entitled APPARATUS FOR MEASURING THE INTRA-OCULAR OR TONOMETRIC PRESSURE OF AN EYE. The content of the latter-noted two patents is totally incorporated hereat by reference.
Arrangements of the type just described and disclosed in the latter-identified patents is inconvenient because they require the intraocular pressure measurement to be made in two steps for each eye. The operator/user must first align the two semicircles and then withdraw the instrument to read the resulting pressure value from the force/pressure scale of the tonometer force applying knob. Not only must this be done twice, once for each eye, but the process is further compromised because dim ambient lighting conditions are required to observe the semicircles under blue cobalt light illumination, and such dim ambient lighting conditions make the subsequent reading of the unilluminated force/pressure scale difficult with attendant built-in inaccuracies.